Four Stroke Engine Operation

Four stroke cycle

Intake stroke

Piston moving downIntake valve openExhaust valve closed

2009/10/05 mmpmm 2

Compression stroke

Piston moving upIntake valve closedExhaust valve closed

Four stroke cycle

2009/10/05 mmpmm 3

Power stroke

Piston moving downIntake valve closedExhaust valve closed

Four stroke cycle

2009/10/05 mmpmm 4

Exhaust stroke

Piston moving upIntake valve closedExhaust valve open

Four stroke cycle

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• Each stroke takes 180° of crankshaft rotation to complete

• All cylinders fire in 720° of crankshaft rotation

• 720 divided by number of cylinders = firing interval

• Odd fire V-6 engine (90° block with 120° rod journals)

Four stroke cycle(Petrol)

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2009/10/05 7

Four stroke cycle(Diesel)

2009/10/05 mmpmm 8

Fuel Injection System

2009/10/05 9

Fuel Pump(Diesel)

Rotary Type Pump Inline Type Pump

2009/10/05 mmpmm 10

Fuel Feed Pump

Draw fuel from tank and feed to injection pump

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Engine Cooling System

Engine heat is transferred through walls of the combustion chambers and through the walls of cylinders

Piston Dwell TimePiston travel is at a minimum

TDC and BDCCrank moves horizontally

Piston velocityMaximum when rod is 90° to crank

AccelerationMaximum 30° earlier

Best VE is obtained by synchronizing valve opening with piston speeds

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Other Valve PositionOverlap

• Both valves are open• End of exhaust & start of intake• Low pressure in exhaust port

Blow down• Exhaust valve opens before BDC• To help evacuate cylinder before piston

reverses• Pumping losses at end of exhaust

stroke

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Valve Mechanism

• Intake valve opening BTDC Low pressure in cylinder Intake valve closing• Intake valve closing ABDC Cylinder pressure is effected by timing• Exhaust valve opening BBDC Residual pressure helps blow down• Exhaust valve closing ATDC Low pressure in exhaust port draws air in

mmpmm 14

Effects On Valve TimingIntake valve opening

Late – Reduced VEEarly – Dilution of intake with exhaust

Intake valve closingLate – Reduces cylinder pressureEarly – Increases cylinder pressure

Exhaust valve openingLate – Pumping lossesEarly – Power reduction

Exhaust valve closingLate – Reduces vacuumEarly – Reduces VE

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COMBUSTIONSpark ignition

Maximum cylinder pressure 15° ATDCTumble and swirlMotion reduces misfiresExcess motion inhibits flowAFR 14.7:1 at part throttle, 12.5:1 under load

Compression ignition18:1 direct injection23:1 pre-chambers for better startingCompression heats to 800-1200 °F

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Valve Mechanism

OHV (overhead valve)Pushrod configurationMany reciprocating partsHigher valve spring pressure requiredCompact engine size compared to OHC

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OHC (overhead cam)

Fewer reciprocating partsReduced valve spring pressure requiredHigher RPM capabilityCylinder head assemblies are taller

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Valve Mechanism

Cam-in-headNo pushrodsUse rocker arms

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Valve Mechanism

Valve lash compensators

Solid liftersNo internal partsPeriodic adjustment

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Hydraulic liftersTo maintain zero lashQuieter No periodic adjustmentAnti-scuff additives are required in oils

Valve lash compensators

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Hydraulic lifter operation

Valve closed• Oil flows through

lifter bore & past check valve

• Plunger return spring maintains zero lash 22

Valve open• Check valve seats

and limits the slippage

• Now operates as a solid lifter

Hydraulic lifter operation

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Hydraulic lifter operation

Return to valve closed• New oil enters the

lifter body• This oil replaces oil

that has leaked between plunger and body (predetermined leakage)

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Metering Device

Metering valve meters the oil flow to the pushrod

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Gear sets• Cam and crank rotate in opposite directions• Noisy if not free of burrs• Helical and spur cut gears

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Timing Gear Wheel sets

Timing chains• Single and double roller• Tension Pulley

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Timing Belt sets

Timing belts• Require maintenance• Silent operation

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Timing Chain Drive

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Engine Lubricating System

Lubrication with oil through pressure to every moving parts

Splash and spray

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Lubricating System

Oil pan baffles• To keep oil in sump during braking,

accelerating, and cornering

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Lubricating System

Oil pan windage tray

• To prevent oil aeration in the sump

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Lubricating System

Oil pumps• Driven by distributors,

gear on camshaft, or crankshaft

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Lubricating System

Oil pumps with pressure relief valves• Gear type pump• Rotor type pump

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Lubricating System

Full flow oil filtering system

• Oil pump output flows through filter first

• Bypass circuit for restricted filters will allow oil to flow to engine

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Lubricating System

Diesel EngineAdvantages

• Higher engine torque• Better fuel economy• Long engine life

Disadvantages

• Engine noise• Exhaust smell• Hard start

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